The present invention relates to cementing operations and, more particularly, in certain embodiments, to methods and cement compositions that utilize treated polyolefin fibers for lost circulation and/or mechanical property enhancement.
In cementing operations, such as well construction and remedial cementing, cement compositions are commonly utilized. Cement compositions may be used in primary cementing operations whereby pipe strings, such as casing and liners, are cemented in well bores. In a typical primary cementing operation, a cement composition may be pumped into an annulus between the exterior surface of the pipe string disposed therein and the walls of the well bore (or a larger conduit in the well bore). The cement composition may set in the annulus, thereby forming an annular sheath of hardened, substantially impermeable material (e.g., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the well bore walls (or to the larger conduit). Among other things, the cement sheath surrounding the pipe string should function to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, such as in squeeze cementing for sealing voids in a pipe string, cement sheath, gravel pack, subterranean formation, and the like.
Once set, the cement sheath may be subjected to a variety of shear, tensile, impact, flexural, and compressive stresses that may lead to failure of the cement sheath, resulting, inter alia, in fractures, cracks, and/or debonding of the cement sheath from the pipe string and/or the formation. This may lead to undesirable consequences such as lost production, environmental pollution, hazardous rig operations resulting from unexpected fluid flow from the formation caused by the loss of zonal isolation, and/or hazardous production operations. Cement failures may be particularly problematic in high temperature wells, where fluids injected into the wells or produced from the wells by way of the well bore may cause the temperature of any fluids trapped within the annulus to increase. Furthermore, high fluid pressures and/or temperatures inside the pipe string may cause additional problems during testing, perforation, fluid injection, and/or fluid production. If the pressure and/or temperature inside the pipe string increases, the pipe may expand and stress the surrounding cement sheath. This may cause the cement sheath to crack, or the bond between the outside surface of the pipe string and the cement sheath to fail, thereby breaking the hydraulic seal between the two. Furthermore, high temperature differentials created during production or injection of high temperature fluids through the well bore may cause fluids trapped in the cement sheath to thermally expand, causing high pressures within the sheath itself. Additionally, failure of the cement sheath also may be caused by forces exerted by shifts in subterranean formations surrounding the well bore, cement erosion, and repeated impacts from the drill bit and the drill pipe.
To counteract these problems associated with the fracturing and/or cracking of the cement sheath, fibers may be included in the cement composition. Various types of fibers have been used heretofore, including those formed of polypropylene, polyester, polyamide, polyethylene, polyolefin, glass, iron, and steel. These fibers may function to control shrinkage cracking in the early stages of the cement setting process, and also may provide resiliency, ductility, and toughness to the set cement composition so that it resists cracking or fracturing. Further, if fracturing or cracking does occur, the fibers may function to hold the set cement composition together, thereby resisting fall back of the cement sheath. Additionally, the fibers may act as lost circulation materials. While polyolefin fibers may be the most preferred in that they are readily available, polyolefin fibers tend to be hydrophobic and difficult to dry blend with cement. The fibers tend to agglomerate in dry cement when it is conveyed causing plugging to occur, and when the cement and fibers are combined with mixing water, the fibers form mates which prevent their dispersion into and throughout the cement composition. The lack of dispersion of the fibers in the cement composition can make it difficult to pump. In some instances, hydrophilic polyolefin fibers have been used in an attempt to improve dry blending, but the hydrophilic polyolefin fibers have not be suitable for dry blending on a commercial scale, for example, due to problems with buildup of blockage in the equipment.